Continuous copolymarization of ethylene with other comonomers is well known in the art. Two types of reactors which are widely used are tubular reactors and stirred tank reactors. Such reactors are quite different in their dimensions, and hence in the environment and state of motion of the reacting mixture. The nature of polymers made from these different types of reactors is described in Armitage, U.S. Pat. No. 4,351,931 for ethylene-methacrylic acid copolymers, which is hereby incorporated by reference.
For the production of random uniform copolymers in a continuous stirred tank reactor, it is desirable to keep the polymerizing mixture in one phase. Increasing the temperature and/or pressure for any monomer mix will decrease the likelihood of separating the mix into two phases. However, when the copolymerization is carried out in internally stirred reactors, the highly polar comonomers result in build up of polymer deposits on the inner surfaces of the reactor far more readily than with non-polar monomers, and at temperatures and pressures at which little or no deposits would occur with non-polar monomers. These deposits may be related to localized phase separation in localized colder regions of the polymerization kettle, such as the monomer inlet region or stirrer seal region.
The formation of these deposits adversely affects the stable operation of the reactor. In addition, with the passage of time, the deposits thermally crosslink to form an intractable gel. As gel particles subsequently are released from the reactor surfaces by the combined action of the shear of the stirrer and the flow of monomers through the vessel, the presence of those crosslinked particles in the copolymer tends to compromise the quality of the copolymer produced.
Comonomers with reactivities substantially equivalent to ethylene, such as vinyl acetate, normally will not exhibit this effect. There, the relative proportion of each monomer incorporated into the polymer is similar to that of the monomer mix. In a continuous reactor operating under steady conditions, this means the composition of the monomer mix will be similar to that of monomers polymerized into the polymer. The polarity of the polymer will tend to be similar to that of the monomer mix, which substantially reduces the likelihood of reactor fouling.
Various methods have been proposed to solve the problem of reactor fouling, including periodic reactor scraping or the addition of an adhesion inhibitor. However, none of the solutions previously suggested provides the desired combination of high production rates and the applicability to comonomers having significantly different rates of reaction.
A need therefore exists for a process for the copolymerization of ethylene with highly reactive comonomers in which a single phase is maintained in the reactor, with the result that polymer separation or deposition on the inner parts of reactor vessel is prevented.